The term "crystalline", used to refer to these materials, relates to the ordered definite crystalline structure of the material which is unique and thus identifiable by a characteristic X-ray diffraction pattern.
The term porous as it refers to such material relates to pores, or channels, of uniform size and channel systems which are uniquely determined by unit structure of the material. The uniform pore size and/or channel systems allow such a material to selectively absorb molecules of certain dimensions and shapes. In the art, microporous material having pores, or channels, of less than 20 Angstroms, can be divided into small, medium and large pore by the diameters of those pores, or channels. The pores of the small pore material have an average diameter of less than 5 Angstroms; medium size pores range from an average diameter of about 5 to about 7 Angstroms, and large pore silicates indicates a diameter of greater than about 7 Angstroms. The word "average" is used to refer to diameter to embrace those species in which the pore is elliptical. Alternatively, the demarcation between small, medium, and large pore materials can be based on the following sorption properties (measured at room temperature for crystallites having a minimum dimension of 0.1 micron):
1. Small pore: n-C.sub.6 /i-C.sub.6 sorption ratio greater than approximately 10.
2. Medium pore: n-C.sub.6 /i-C.sub.6 is less than 10 and n-C.sub.6 /mesitylene sorption ratio greater than approximately 5.
3. Large pore: n-C.sub.6 /mesitylene sorption ratio less than approximately 5.
In the art, zeolites are a subclass of crystalline porous silicates. Zeolites can contain aluminum as well as silicon. In some zeolites, the upper limit of the silicon/aluminum atomic ratio is unbounded. ZSM-5 is one such example wherein the silicon/aluminum atomic ratio is at least 2.5 and up to infinity. By way of illustration, U.S. Pat. No. 3,941,871, reissued as RE 29,948, discloses a porous crystalline silicate made from a reaction mixture containing no deliberately added aluminum and exhibiting the X-ray diffraction pattern characteristic of ZSM-5 zeolites; in certain examples tin is deliberately added to the silicate synthesis mixture.
Zeolites can be acidic or non-acidic, depending on the framework aluminum content and on the amount of compensating cations, such as Na.sup.+, K.sup.+, etc. Acidity is attributable to, for example, the hydroxyl groups bonded to the framework aluminum of the zeolite.